[Clang][Driver] Recognize the AIX OBJECT_MODE environment setting

[Clang][Driver] Recognize the AIX OBJECT_MODE environment setting

Summary:
AIX uses an environment variable called OBJECT_MODE to indicate to
utilities in the toolchain whether they should be operating in 32-bit or
64-bit mode. This patch makes the clang driver recognize the current
OBJECT_MODE setting when we are operating with an AIX target and adds a
custom diagnostic for invalid settings.

For more details about OBJECT_MODE on AIX see:

https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.3/com.ibm.xlc1313.aix.doc/compiler_ref/tusetenv1.html
https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.3/com.ibm.xlc1313.aix.doc/compiler_ref/opt_3264.html

Reviewers: stevewan, hubert.reinterpretcast, ShuhongL, jasonliu

Reviewed By: hubert.reinterpretcast, jasonliu

Subscribers: jasonliu, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82476